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Investment Securities (Investment Holdings Fixed Maturity Table) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017
U.S. Government and agency obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | $ 127.0 | $ 297.8
Trading Securities, Unrealized Holding Gain | 0.0 | 0.0
Trading Securities, Unrealized Holding Loss | (1.6) | (1.3)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 125.4 | 296.5
Debt securities issued by corporations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 509.4 | 867.6
Trading Securities, Unrealized Holding Gain | 0.2 | 2.9
Trading Securities, Unrealized Holding Loss | (9.6) | (4.3)
Net foreign currency gains (losses) | 0.0 | 14.7
Fixed maturity investments, at fair value | 500.0 | 880.9
Municipal obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 281.1 | 252.0
Trading Securities, Unrealized Holding Gain | 0.4 | 3.7
Trading Securities, Unrealized Holding Loss | (3.5) | (0.8)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 278.0 | 254.9
Mortgage-backed and asset-backed securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 132.4 | 697.2
Trading Securities, Unrealized Holding Gain | 0.0 | 1.6
Trading Securities, Unrealized Holding Loss | (3.9) | (4.1)
Net foreign currency gains (losses) | 0.0 | 0.0
Fixed maturity investments, at fair value | 128.5 | 694.7
Foreign government, agency and provincial obligations |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost |  | 2.6
Trading Securities, Unrealized Holding Gain |  | 0.0
Trading Securities, Unrealized Holding Loss |  | 0.0
Net foreign currency gains (losses) |  | 0.1
Fixed maturity investments, at fair value |  | 2.7
Fixed Income Investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Cost or amortized cost | 1,049.9 | 2,117.2
Trading Securities, Unrealized Holding Gain | 0.6 | 8.2
Trading Securities, Unrealized Holding Loss | (18.6) | (10.5)
Net foreign currency gains (losses) | 0.0 | 14.8
Total fixed maturity investments including assets held for sale | $ 1,031.9 | $ 2,129.7

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